In general, the present invention pertains to electrolytic chlor-alkali membrane cells wherein fluoropolymer membranes are disposed between anodes and cathodes, thereby separating catholyte portions from corresponding anolyte portions. Such membranes are capable of transporting electrolysis ions while being substantially hydraulically-impermeable as opposed to the historically popular diaphragms (usually asbestos) which permit limited, but substantial, flow-through of the aqueous electrolyte solution.
More particularly, the present invention pertains to such membrane cells wherein the catholyte liquor flows sequentially from cell-to-cell (also called "series flow") as the anolyte liquor (brine) flows simultaneously through each of the anolyte portions (also called "parallel flow"). Also more particularly, the present invention pertains to the use of fluoropolymer membranes which have pendant functional groups on the cathode side which are less hydrophilic than the currently popular fluoropolymers having pendant fluorosulfonic acid groups.
It is known in the art that, when using membranes, a substantially salt-free caustic solution may be removed as the catholyte from the chlor-alkali cell. It is also known that water, or a weak caustic solution, may be initially fed to the first catholyte portion of the bank of membrane cells, then fed serially (sequentially) to one or more additional catholyte portions, gaining in caustic strength from cell-to-cell. This series flow (sometimes called "cascade flow") is taught, e.g., in U.S. Pat. Nos. 1,284,618; 4,057,474; and 4,076,603. These patents teach series catholyte flow as a means to increase caustic current efficiency when membranes are used that suffer a decrease in current efficiency as caustic strength is increased.
It has been reported that a "perfluorocarboxylic acid type membrane" gives excellent performance in comparison to a perfluoro sulfonic acid membrane sold under the trademark "Nafion" by E. I. duPont de Nemours and Company. This was reported by Maomi Seko or Asahi Chemical Industry Co., Ltd. in a paper titled Commercial Operation of The Ion Exchange Membrane Chlor-Alkali Process presented to The American Chemical Society Centennial Meeting at New York on April 4-9, 1976. The same author presented a subsequent paper titled The Asahi Chemical Membrane Chlor-Alkali Process to The Chlorine Institute, Inc., 20th Chlorine Plant Managers' Seminar at New Orleans, La. on Feb. 9, 1977, in which he defined "perfluorocarboxylic acid membrane" as having the chemical structure ##STR1## which is said to be less hydrophilic than perfluorosulfonic acid membranes.
Among the various Nafion.RTM. types of fluoropolymer membranes which are said to be "perfluorosulfonic acid membranes" are those which have been modified by reaction with an organic amine (e.g. butyl amine) to convert at least part of the fluorosulfonic acid groups to sulfonamide groups. These sulfonamide groups are, like the perfluorocarboxylic acid groups described above, less hydrophilic than the perfluorosulfonic acid groups and therefore have greater resistance to back-migration of OH.sup.- ions from catholyte to anolyte. It is also known that fluoropolymer films containing functional sulfonyl fluoride groups can be post-treated with difunctional amines, such as ethylene diamine, to reduce the hydrophilicity of the functional groups by forming the sulfonamide group and by crosslinking the polymer. A pre-treatment of such fluoropolymer, before forming it into the desired film, substantially reduces the solubility of the fluoropolymer and increases the softening temperature usually making the film-formation very difficult, if not impossible.
The fluoropolymer membranes of interest in the present invention are those which have greater hydrophilicity than polytetrafluoroethylene (Teflon.RTM.) polymers alone, but which, on at least the cathode side, have less hydrophilicity than perfluorosulfonic acid membranes wherein the hydrophilic substituent of the fluoropolymer contains sulfonic acid groups, [HO--SO.sub.2 --].
There are several interrelationships between various properties of a fluoropolymer membrane to be taken into consideration, such as:
1. The thicker the membrane (of a given composition) the greater is the electrical resistance through the membrane;
2. For a given thickness of the membrane the greater the population of functional groups, the greater is the electrical conductivity through the film; a functional group of, say, 1000 eq. wt. has more groups per mol. wt. unit than does one with, say, 1500 eq. wt.
3. For a given thickness of membrane, one with 1000 eq. wt. of functional groups is more hydrophilic than one with, say 1500 eq. wt. of the same functional group;
4. Strongly hydrophilic groups, such as sulfonic acid groups, reach a high degree of hydration in an operating cell, thus allowing penetration of the product hydroxide ion into the absorbed water surrounding the ion exchange site and this, in turn, leads to loss in caustic efficiency. At catholyte caustic strengths of about 18% or more, the performance of the membrane decreases with time; at less than about 18% caustic concentration, the performance of the membrane (current efficiency) is substantially stable with time. Decreasing the population of functional groups in the membrane causes a decrease in the water absorbed per functional group which in turn leads to higher caustic efficiency. Both the decrease in the number of functional groups and less hydration per functional group lead to an increase in electrical resistance (giving an increase in cell voltage);
5. Crosslinking of polymers containing sulfonic acid functional groups decreases the amount of water absorbed per functional group without suffering the attendant increase in voltage which would accompany a decrease in the number of functional groups. There is, however, a voltage increase associated with decreasing the water absorbed per functional group;
6. Employing carboxylic acid groups or sulfonamide groups in place of an equivalent number of sulfonic acid groups, provides less hydrophilicity per functional group. The number of functional groups can be increased so as not to suffer an appreciable increase in electrical resistance while still maintaining the water absorbed per functional group at a low enough level to achieve high caustic efficiency;
7. Theoretically, a membrane containing functional ion exchange groups should be operable with a monomolecular thickness, however such thin films would not have the strength and physical integrity required in practical applications. Therefore, it is necessary to employ, as membranes, sheets that have a thickness, generally, of at least about 2 mils and as much as 20 mils, the thickness being that of a sole sheet, a reinforced sheet, or a plurality of laminated sheets. For instance, a reinforced laminated sheet can be prepared from two films, each of about, say, 0.5 to 10 mils thickness, sandwiched together with a porous, reinforcing scrim in between them. Generally such sandwiched films are heat-plastified to effect bonding and the bonding process causes the two outside films to bond together at points of contact in the void spaces of the porous scrim. The overall thickness of the laminate is decided primarily by the thicknesses of the components, and somewhat by the pressure applied during bonding.